Chemical sensitizing of photographic silver halide emulsions



N Drawing. Filed July 30, 1957, Ser. No. 675,043 Claims. (Cl. 96-107)This invention relates to photographic materials and particularly tolight-sensitive silver halide emulsion containing a reaction product ofgelatin and thioformamide as a sensitizing agent for the purpose ofincreasing the speed and contrast.

Photographic silver halide emulsions are usually prepared by mixing asoluble silver salt such as silver nitrate with a soluble alkali metalhalide such as sodium chloride, potassium bromide or potassium iodide inthe presence of a colloid, particularly gelatin. The selection ofgelatin for this purpose is very critical and requires very carefultesting because the photographic properties of the finished emulsion,namely, speed, contrast, freedom from fog and storage stability dependon the presence of minute quantities of active agents in the gelatin.Some of these substances, commonly referred to as ripeners orsensitizers,

impart to the emulsion a ripening or sensitizing effect and increase thespeed and contrast.

The gelatins with which the emulsion work are more often than notunsatisfactory because they contain too much or too little of thesesensitizers. This is understandable because gelatin-is derived fromvarious natural sources including the hides, bones, tendons and sinewsof cattle and pigs. Depending on the source of the raw material and onthe process selected for the manufacture and extraction, the gelatinswill contain varying amounts of these substances.

The nature or chemical structure of the active gelatin ingredients andtheir actual configuration is far from certain although it is nowgenerally assumed that sulfurcontaining compounds play a vital part inthenaturalcations, particularly the formation of fog so dense that it isbeyond control by the usual antifogging agents.

1 have found that the reaction products of gelatin and thioformamidepossess valuable ripening and sensitizing properties and have theability, when present in photographic emulsions, to increase theirspeed, contrastor both. 7

The use of the reaction products of gelatin and thioformamide in thepreparation of photographic silver halide emulsion, the process ofpreparing said silver halide emulsions and the silver halide emulsionsobtained thereby constitute, accordingly, the purposes and objects ofthe present invention.

' In carrying out the invention, the thioformamide derivative of gelatinis dissolved in water and the resultant solution is added to the gelatinor other colloidal carrier used in the preparation of the emulsion at astage where the effectiveness of the derivative is most pronounced." Ingeneral, this is the case where normally gelatin is added tophotographic emulsion. Accordingly, this derivative can be added to thegelatin substitute which is to be used for the initial preparation ofthe photographic emulsion or it chemist must atent Patented Sept. 12,1961 can be used with the gelatin which is added at the ripening stage;7

The thioformamide derivatives of gelatin are prepared according to themethod described by-F. Micheel et al. in Chemische Berichte, vol. 80,pages 37-38 (1947), and vol. 82, page 136 (1949). It is believed thatthe reaction product has the following general structure:

bears a strong resemblance to those natural gelatins which haveexcellent ripening properties. The available sulfur or other ripeningbody is released gradually and when used in the proper concentrationsand conditions does not give rise to the formation of fog.

As mentioned above, the thioformamides are preferably reacted with inertgelatin. Similarly, the reaction products thus obtained are ofparticular utility when used with inert gelatin. The latter aregenerally prepared by removing the active agents from a commerciallyavailable gelatin, for instance, by prolonged washing or by treatmentwith charcoal, hydrogen peroxide or peracetic acid as described by H.Ammann-Brass in Science et Industries 7 Photographiques, vol. 25, page491 (1954).

The light-sensitive silver salt present in the silver halide 1 materialscontemplated herein may be silver bromide, silver iodide, silverchloride or mixed silver halides. The sensitizing silver salt, moreover,may be present in any of the usual photographic carriers or colloidssuch as gelatin, collodion, albumen, casein, polyvinyl alcohol and thelike. In addition, the light-sensitive silver halide materials maycontain any of the various ingredients of photographic emulsions, e.g.,sensitizing dyes, stabilizers or fog inhibitors, hypersensitizers andsurfactants.

Finally, the silver halide materials may be those used for producingblack and white images or for producing color images particularly thosesilver halide emulsions,

which contain a color forming component.

The particular quantities of the thioforInamide-gel-atin derivatives arecritical to the extent that the quantities needed will vary withdifferent types of emulsions. From 20 to 200 milligrams of reactionproduct per kilogram of colloidal carrier solids have been found mostsatisfactory.

I have further found that optimum results are obtained by utilizing aratio of silver halide to colloidal carrier to thioformamide on thefollowing order:

450 to 750 parts of silver halide 500 to 1000 parts of the colloidalcarrier 0.02 to 0.2 part of thioformamide gelatin derivative Theseratios are expressed in terms of parts by weight.

3 PREPARATION A reaction product of gelatin with thioformamide isprepared by adding 4 grams of thioformarnide to 101 grams of inertgelatin and kneading the mixture thoroughly for six hours until thereaction product is a highly viscous sticky mass. It is washed severaltimes with ether and alcohol in order to remove the excess ofthioformamide, and then shaken for two days with alcohol and chloroformand dried. The solid product obtained has a sulfur content of about 4.5percent.

Example I Seventy-five grams of potassium bromide, 4 grams of potassiumiodide and 5 grams of gelatin are dissolved in 200 mls. of distilledwater. To this solution is added 0.15 cc. of a 0.1 percent solution ofthe thioforrnamide gelatin reaction product. The temperature is adjustedto 65 C., and maintained there throughout the mixing operation. Asolution of 90 grams of silver nitrate in 800 mls. of water is added tothe potassium bromide-iodide gelatin solution over a period of 20minutes under continuous stirring. At this stage, 100 grams of drygelatin and mls. of 0.1% solution of the thioforrnamide gelatin reactionproduct are added. The mixture is digested for 20 minutes to allow theadded gelatin to dissolve. The emulsion is then poured into a shallowpan and allowed to cool. After the emulsion has set to a firm gel, it iscut into cubes and washed, until the inorganic salts are removed. Thenthe emulsion is re-melted and submitted to another ripening process.During this process, 20 grams dry strength gelatin are added and also 2cc. of a 0.1% solution of thiofor-mamide treated gelatin. Thetemperature during the ripening is held at 50 C. The usual coatingfinals, including stabilizers and surfactants are added and the emulsionis coated on a cellulose ester base and dried.

Samples of these film coatings were then exposed in a Type 113Sensitometer and developed in a developer of the following composition:

Grams Metol 1.5 Sodium sulfite, anhydrous- 45 Sodium bisulfite lHydroquinone r 3 Sodium carbonate, monohydrated 6 Potassium bromide -8Water to make 1 liter.

The developed was short-stopped, fixed, washed and dried. The filmprepared from the emulsion containing the gelatin-thiofornramidereaction product was cornpared with another film prepared and processedin the identical manner except that the thioformamide reaction producthad been omitted.

Expressed in numerical terms, the following results were obtained:

Relative Gamma Fog Speed With thioformamide gelatin 100 0. 86 0.08Without thioformamide gelatin. 0. 15 0.06

Example II grams gelatin dissolved in 300 cc. of water are added and thebatch is digested for a period of 25 minutes. The pH of the emulsion isadjusted to 6.8. During the mixing and digestion, the temperature isheld at 71 C. The emulsion is then processed as described in Example -Iand is finally submitted to a second ripening process, for which purposethe washed emulsion is melted slowly, 0.5 gram of thegelatin-thiofonmamide reaction product are added and the mixture isheated to 40 C. and kept at this temperature for 2 hours. The emulsionis then coated and processed as described in Example I. The resultsobtained were similar to those described in Example I and the followingreadings were obtained:

Relative Gamma Fog Speed Emulsion with the thioformamido gel preparedfrom inert gelatin 70 0.75 0. 09 Emulsion without thioformamide gelatinreaction l0 0. 15 0. Q6

in the presence of a sensitizing amount of a reaction product preparedby kneading a mixture of 10 parts by Weight of essentially inert solidgelatin and from 2 to 6 parts by weight of thioformamide until a highlyviscous sticky mass is obtained.

2. A process as recited in claim 1 wherein the colloidal carrier isgelatin essentially free from natural ripening substances.

3. The process of increasing the speed of a photographic silver halideemulsion which comprises, digesting an emulsion of silver halide in anessentially inert colloidal carrier in the presence of '20 to 200milligrams per kilogram of said colloidal carrier of a reacton productprepared by kneading a mixture of 10 parts by weight of essentiallyinert solid gelatin and from 2 to 6 parts by weight of thioformamideuntil a highly viscous sticky mass of reaction product is obtained.

4. A process as recited in claim 3 wherein the colloidal carrier isgelatin essentially free from natural ripening substances.

5. The process of increasing the speed of a photographic silver halideemulsion which comprises, digesting an emulsion of silver halide in anessentially inert colloidal carrier in the presence of a reactionproduct prepared by kneading a mixture of 1 0 par-ts by weight ofessentially inent solid gelatin and horn 2 to 6 parts by weight ofthioformamide until a highly viscous sticky mass of reaction product isobtained, the amount of colloidal carrier in the emulsion ranging from500 to 1000 parts by weight, the amount of silver halide ranging from450 to 750 parts by weight and the amount of said reaction productranging from 0.02 to 0.2 part by weight.

References Cited in the file of this patent UNITED STATES PATENTSSheppard Mar. 2, 1926 Sheppard et al. Apr. 5, 1927 OTHER REFERENCES

1. THE PROCESS OF INCREASING THE SPEED OF A PHOTOGRAPHIC SILVER HALIDE EMULSION WHICH COMPRISES, DIGESTING AN EMULIN THE PRESENCE OF A SENSITIZING AMOUNT OF A REACTION PRODUCT PREPARED BY KNEADING A MIXTURE OF 10 PARTS BY WEIGHT OF ESSENTIALLY INERT SOLID GELATIN AND FROM 2 TO 6 PARTS BY WEIGHT OF THIOFORMAMIDE UNTIL A HIGHLY VISCOUS STICKY MASS IS OBTAINED. 